diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
index 0000000..8996490
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,19 @@
+# Änderungsprotokoll
+
+## 2026-09-18
+
+Seit Commit `0eb32fb` wurden folgende Änderungen vorgenommen:
+
+- Neue geschützte Admin-Seite `author_price.php` für die Berechnung von Autorenpreisen.
+- Printbücher können per Dropdown ausgewählt werden.
+- Berechnung des Autorenpreises auf Grundlage von Brutto-Preis, 7 % Mehrwertsteuer, Druckkosten und einem frei wählbaren Rabatt.
+- Der Rabatt ist standardmäßig mit 85 % vorbelegt und kann zwischen 0 und 100 % eingegeben werden.
+- Die Ersparnis pro Exemplar wird separat ausgewiesen.
+- Der Autorenrabatt wird als eigener Rechenposten angezeigt.
+- Checkbox ergänzt, um alternativ die kumulierte Autorenmarge aller am Buch beteiligten Autoren als Rabatt zu verwenden.
+- Das manuelle Rabattfeld wird bei aktivierter Margen-Checkbox deaktiviert.
+- Die kumulierte Autorenmarge wird aus allen Buch-Autor-Zuordnungen berechnet.
+- JavaScript für die unmittelbare Aktivierung und Deaktivierung des Rabattfelds ergänzt.
+- Navigation im Admin-Bereich um den Link zum Autorenpreis-Rechner erweitert.
+- CSS für die Anordnung von Rabattfeld und Checkbox ergänzt.
+- PHP-, SQL- und Zugriffsschutz-Prüfungen für die neue Funktion durchgeführt.
diff --git a/admin.php b/admin.php
index b1d57f6..50e1e24 100644
--- a/admin.php
+++ b/admin.php
@@ -156,7 +156,7 @@ $token = csrf();
 ?><!doctype html>
 <html lang="de"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width,initial-scale=1"><title>Autorenmargen – Admin</title><link rel="stylesheet" href="css/admin.css"></head>
 <body data-admin-app data-csrf="<?= e($token) ?>" data-author-search="author_search.php">
-<header><h1>Autorenmargen – Admin</h1><nav><a class="button secondary" href="billing.php">Abrechnung erstellen</a><form method="post" class="inline-form"><input type="hidden" name="action" value="logout"><input type="hidden" name="csrf_token" value="<?= e($token) ?>"><button class="secondary" type="submit">Abmelden</button></form></nav></header>
+<header><h1>Autorenmargen – Admin</h1><nav><a class="button secondary" href="author_price.php">Autorenpreis-Rechner</a><a class="button secondary" href="billing.php">Abrechnung erstellen</a><form method="post" class="inline-form"><input type="hidden" name="action" value="logout"><input type="hidden" name="csrf_token" value="<?= e($token) ?>"><button class="secondary" type="submit">Abmelden</button></form></nav></header>
 <main><?php if ($message): ?><p class="notice"><?= e($message) ?></p><?php endif; ?><?php if ($error): ?><p class="notice error"><?= e($error) ?></p><?php endif; ?>
 <div class="grid">
 <section class="panel"><h2><?= $author['id'] ? 'Autor bearbeiten' : 'Autor anlegen' ?></h2><form method="post"><input type="hidden" name="action" value="save_author"><input type="hidden" name="csrf_token" value="<?= e($token) ?>"><input type="hidden" name="author_id" value="<?= e($author['id']) ?>"><label for="author-name">Name</label><input id="author-name" name="name" maxlength="255" value="<?= e($author['name']) ?>" required><label for="address">Adresse</label><input id="address" name="address" maxlength="255" value="<?= e($author['address']) ?>"><label for="postal-code">PLZ</label><input id="postal-code" name="postal_code" maxlength="20" value="<?= e($author['postal_code']) ?>"><label for="city">Ort</label><input id="city" name="city" maxlength="120" value="<?= e($author['city']) ?>"><label for="bank-account">IBAN</label><input id="bank-account" name="bank_account" maxlength="255" value="<?= e($author['bank_account']) ?>"><label for="bic">BIC</label><input id="bic" name="bic" maxlength="20" value="<?= e($author['bic']) ?>"><label for="bank-name">Bankname</label><input id="bank-name" name="bank_name" maxlength="255" value="<?= e($author['bank_name']) ?>"><label for="paypal-address">PayPal-Adresse</label><input id="paypal-address" name="paypal_address" maxlength="255" value="<?= e($author['paypal_address']) ?>"><div class="actions"><button type="submit">Speichern</button><?php if ($author['id']): ?><a class="button secondary" href="admin.php">Abbrechen</a><?php endif; ?></div></form></section>
diff --git a/author_price.php b/author_price.php
new file mode 100644
index 0000000..2f17e98
--- /dev/null
+++ b/author_price.php
@@ -0,0 +1,154 @@
+<?php
+
+declare(strict_types=1);
+
+require_once __DIR__ . '/inc/admin_bootstrap.php';
+requireAdmin();
+
+const AUTHOR_PRICE_VAT_RATE = 0.07;
+const DEFAULT_AUTHOR_DISCOUNT_PERCENT = 85.0;
+
+function authorPriceMoney(float $value): string
+{
+    return number_format($value, 2, ',', '.') . ' €';
+}
+
+$books = $pdo->query(
+    'SELECT b.id, b.title, b.selling_price, b.printing_costs, b.isbn_print,
+            COALESCE(SUM(ba.margin_percent), 0) AS author_margin_percent
+     FROM books b
+     LEFT JOIN book_authors ba ON ba.book_id = b.id
+     WHERE b.isbn_print IS NOT NULL AND b.isbn_print <> ""
+     GROUP BY b.id
+     ORDER BY b.title'
+)->fetchAll();
+
+$selectedId = filter_var($_GET['book_id'] ?? null, FILTER_VALIDATE_INT, ['options' => ['min_range' => 1]]) ?: null;
+$useAuthorMargin = ($_GET['use_author_margin'] ?? '') === '1';
+$discountInput = $_GET['discount_percent'] ?? (string) DEFAULT_AUTHOR_DISCOUNT_PERCENT;
+$discountPercent = filter_var($discountInput, FILTER_VALIDATE_FLOAT);
+$discountError = $discountPercent === false || $discountPercent < 0 || $discountPercent > 100;
+if ($discountError) {
+    $discountPercent = DEFAULT_AUTHOR_DISCOUNT_PERCENT;
+}
+$discountRate = (float) $discountPercent / 100;
+$retainedRate = 1 - $discountRate;
+$selectedBook = null;
+foreach ($books as $book) {
+    if ((int) $book['id'] === $selectedId) {
+        $selectedBook = $book;
+        break;
+    }
+}
+
+$calculation = null;
+if ($selectedBook !== null) {
+    if ($useAuthorMargin) {
+        $discountPercent = (float) $selectedBook['author_margin_percent'];
+        $discountError = false;
+    }
+    $discountRate = (float) $discountPercent / 100;
+    $retainedRate = 1 - $discountRate;
+    $grossPrice = (float) $selectedBook['selling_price'];
+    $printingCosts = (float) $selectedBook['printing_costs'];
+    $netPrice = $grossPrice / (1 + AUTHOR_PRICE_VAT_RATE);
+    $revenueBeforeAuthorDiscount = $netPrice - $printingCosts;
+    $authorDiscount = $revenueBeforeAuthorDiscount * $discountRate;
+    $retainedRevenue = $revenueBeforeAuthorDiscount * $retainedRate;
+    $authorPriceNet = $printingCosts + $retainedRevenue;
+    $authorPriceVat = $authorPriceNet * AUTHOR_PRICE_VAT_RATE;
+    $authorPriceGross = $authorPriceNet + $authorPriceVat;
+    $saving = $grossPrice - $authorPriceGross;
+
+    $calculation = [
+        'gross_price' => $grossPrice,
+        'net_price' => $netPrice,
+        'printing_costs' => $printingCosts,
+        'revenue_before_author_discount' => $revenueBeforeAuthorDiscount,
+        'author_discount' => $authorDiscount,
+        'discount_percent' => (float) $discountPercent,
+        'retained_percent' => $retainedRate * 100,
+        'use_author_margin' => $useAuthorMargin,
+        'author_margin_percent' => (float) $selectedBook['author_margin_percent'],
+        'retained_revenue' => $retainedRevenue,
+        'author_price_net' => $authorPriceNet,
+        'author_price_vat' => $authorPriceVat,
+        'author_price_gross' => $authorPriceGross,
+        'saving' => $saving,
+    ];
+}
+
+$token = csrf();
+?><!doctype html>
+<html lang="de">
+<head>
+    <meta charset="utf-8">
+    <meta name="viewport" content="width=device-width,initial-scale=1">
+    <title>Autorenpreis-Rechner</title>
+    <link rel="stylesheet" href="css/admin.css">
+</head>
+<body>
+<header>
+    <h1>Autorenpreis-Rechner</h1>
+    <nav>
+        <a class="button secondary" href="admin.php">Stammdaten</a>
+        <a class="button secondary" href="billing.php">Abrechnung</a>
+        <form method="post" action="admin.php" class="inline-form">
+            <input type="hidden" name="action" value="logout">
+            <input type="hidden" name="csrf_token" value="<?= e($token) ?>">
+            <button class="secondary" type="submit">Abmelden</button>
+        </form>
+    </nav>
+</header>
+<main>
+    <section class="panel">
+        <h2>Buch auswählen</h2>
+        <form method="get">
+            <label for="book-id">Printbuch</label>
+            <select id="book-id" name="book_id" required>
+                <option value="">Bitte auswählen</option>
+                <?php foreach ($books as $book): ?>
+                    <option value="<?= (int) $book['id'] ?>"<?= (int) $book['id'] === $selectedId ? ' selected' : '' ?>><?= e($book['title']) ?> (<?= e($book['isbn_print']) ?>)</option>
+                <?php endforeach; ?>
+            </select>
+            <div class="discount-input-row">
+                <div>
+                    <label for="discount-percent">Autorenrabatt (%)</label>
+                    <input id="discount-percent" type="number" name="discount_percent" min="0" max="100" step="0.01" value="<?= e(number_format((float) $discountPercent, 2, '.', '')) ?>"<?= $useAuthorMargin ? ' disabled' : '' ?> required>
+                </div>
+                <label class="checkbox-label" for="use-author-margin"><input id="use-author-margin" type="checkbox" name="use_author_margin" value="1"<?= $useAuthorMargin ? ' checked' : '' ?>> Kumulierte Autorenmarge als Rabatt verwenden</label>
+            </div>
+            <?php if ($discountError): ?><p class="notice error">Der Rabatt muss zwischen 0 und 100 Prozent liegen. Es wurde 85 Prozent verwendet.</p><?php endif; ?>
+            <p class="muted">Hinterlegte Autorenmarge für dieses Buch: <?= number_format((float) ($selectedBook['author_margin_percent'] ?? 0), 2, ',', '.') ?> %</p>
+            <div class="actions"><button type="submit">Preis berechnen</button></div>
+        </form>
+    </section>
+
+    <?php if ($calculation !== null): ?>
+        <section class="panel">
+            <h2>Autorenpreis: <?= e($selectedBook['title']) ?></h2>
+            <p class="muted">Berechnung pro Exemplar. Die Zwischenwerte werden ungerundet berechnet und erst für die Anzeige auf Cent gerundet.</p>
+            <div class="table-wrap">
+                <table class="billing-table">
+                    <tbody>
+                        <tr><th>Regulärer Brutto-Preis</th><td><?= authorPriceMoney($calculation['gross_price']) ?></td></tr>
+                        <tr><th>Regulärer Netto-Preis (Brutto ÷ 1,07)</th><td><?= authorPriceMoney($calculation['net_price']) ?></td></tr>
+                        <tr><th>Abzüglich Druckkosten</th><td><?= authorPriceMoney($calculation['printing_costs']) ?></td></tr>
+                        <tr><th>Erlös vor Autorenrabatt</th><td><?= authorPriceMoney($calculation['revenue_before_author_discount']) ?></td></tr>
+                        <tr><th>Autorenrabatt <?= number_format($calculation['discount_percent'], 2, ',', '.') ?> %<?= $calculation['use_author_margin'] ? ' (kumulierte Autorenmarge)' : '' ?></th><td><?= authorPriceMoney($calculation['author_discount']) ?></td></tr>
+                        <tr><th>Verbleibender Erlösanteil <?= number_format($calculation['retained_percent'], 2, ',', '.') ?> %</th><td><?= authorPriceMoney($calculation['retained_revenue']) ?></td></tr>
+                        <tr><th>Autorenpreis netto (Druckkosten + verbleibender Anteil)</th><td><?= authorPriceMoney($calculation['author_price_net']) ?></td></tr>
+                        <tr><th>Mehrwertsteuer 7 % auf Autorenpreis</th><td><?= authorPriceMoney($calculation['author_price_vat']) ?></td></tr>
+                        <tr><th>Autorenpreis brutto</th><td><strong><?= authorPriceMoney($calculation['author_price_gross']) ?></strong></td></tr>
+                        <tr><th>Ersparnis pro Exemplar</th><td><strong><?= authorPriceMoney($calculation['saving']) ?></strong></td></tr>
+                    </tbody>
+                </table>
+            </div>
+        </section>
+    <?php elseif ($selectedId !== null): ?>
+        <section class="panel"><p class="notice error">Das ausgewählte Buch ist nicht als Printbuch vorhanden.</p></section>
+    <?php endif; ?>
+</main>
+<script src="scripts/author_price.js"></script>
+</body>
+</html>
diff --git a/css/admin.css b/css/admin.css
index 86ee454..ad32e74 100644
--- a/css/admin.css
+++ b/css/admin.css
@@ -31,6 +31,10 @@ th { background: #e0e7ff; color: #172554; }
 nav { display: flex; align-items: center; gap: .6rem; flex-wrap: wrap; }
 nav a { color: #fff; }
 .billing-table input[type=number] { min-width: 7rem; }
+.discount-input-row { display: flex; gap: 1rem; align-items: end; flex-wrap: wrap; }
+.discount-input-row > div { flex: 1 1 15rem; }
+.checkbox-label { display: flex; align-items: center; gap: .45rem; padding-bottom: .65rem; }
+.checkbox-label input { width: auto; }
 .muted { color: #64748b; }
 .visually-hidden { position: absolute; width: 1px; height: 1px; padding: 0; margin: -1px; overflow: hidden; clip: rect(0, 0, 0, 0); white-space: nowrap; border: 0; }
 @media (max-width: 600px) { .author-row { grid-template-columns: 1fr; } header { align-items: flex-start; flex-direction: column; } }
diff --git a/scripts/author_price.js b/scripts/author_price.js
new file mode 100644
index 0000000..f569cc9
--- /dev/null
+++ b/scripts/author_price.js
@@ -0,0 +1,12 @@
+(() => {
+  const checkbox = document.querySelector('#use-author-margin');
+  const discountInput = document.querySelector('#discount-percent');
+  if (!checkbox || !discountInput) return;
+
+  const updateDiscountInput = () => {
+    discountInput.disabled = checkbox.checked;
+  };
+
+  checkbox.addEventListener('change', updateDiscountInput);
+  updateDiscountInput();
+})();
\ No newline at end of file